1. Field of the Invention
The present invention relates to a color thermal printing system having a print head assembly which is insensitive to disturbances of the cover mechanism for the thermal printer.
2. Description of the Prior Art
Thermal printers often provide a means to replace the thermal dye donor media and/or the dye receiver media in the printer by opening a portion of the printer. To open the printer, the user releases a latch in a cover mechanism and swings the cover open to gain access to the media supplies. These cover mechanisms are usually located in the top portion of the printer, opening upward.
The cover mechanism can have some of the major printer components attached to it, so that as the cover is swung open these components move away from media transport paths to permit easier loading of new media. The cover mechanism usually has a hinge or pivot axis along one side of the mechanism and a latch component to keep the cover closed during operation but permit opening for loading media. To permit proper latch operation and opening, the cover cannot be rigidly locked to the rest of the thermal printer structure. These configurations have a problem when a weight, such as the user leaning on the cover or a heavy load like a book, is placed upon the cover. These weights distort the cover and move components attached to the cover even though the cover is closed. When the print head is moved, artifacts or defects can be produced in a print.
FIG. 1 shows a prior art thermal printer 10 (see U.S. Pat. No. 4,838,713) where a platen 20 and a receiver supply spool 22 are attached to a cover mechanism 30. The cover mechanism 30 is attached in turn to a main printer structure 12 at cover pivot shaft 32. A print head assembly 70 which mounts a print head 80 and a head positioning mechanism 50 are attached to the main printer structure 12 at a head pivot shaft 52. When the cover mechanism 30 is opened, the platen 20 lifts away from the print head assembly 70. The cover pivot shaft 32 is widely separated from the head pivot shaft 52. When force is applied to the cover mechanism 30, a misalignment of the platen 20 and the print head 80 occurs which is directly related to the separation of the cover and head pivot shafts 32, 52. Such misalignment results in inferior print quality.
Alternate designs for thermal printers reverse the component configurations, attaching the print head assembly to the cover while the platen is attached to the main printer structure. However, these designs also have widely separated pivot shafts for the cover and print head assemblies. All of these thermal printer designs are thus sensitive to disturbance of the cover mechanism caused by, for example, printer vibration. These disturbances result in misalignment of the print head and the platen, producing lower quality prints. The cover mechanism's sensitivity to disturbance is directly related to the distance between the pivot shaft of the cover mechanism and the pivot shaft of the component attached to the cover mechanism, whether the print head assembly or the platen.